I agree with Boeing Nut and ElPremier on this one...
The 737-900 is a great aircraft to fill customer's desires for a 737 bigger than the -800 (carrying 15 more passengers), just a ton
of cargo room as ElPremier said, and a little shorter range.
This fits the bill for launch customer Alaska Airlines (received the worlds first -900 last year... reallly nice plane, btw), which has ordered 13 (by my count) so far.
Take a look at their (all-Boeing
and you'll see that each aircraft fills a specific role for Alaska. You've got the -900 for heavily-travelled routes and big cargo runs, the -700 for long range flights (ie ANC-ORD, SEA-IAD, SEA-DCA, soon to LAX-Cancun), the -400 and -83 for the regular run-of-the-mill flights, and the -200C for inter-Alaska/heavy cargo flights.
I think the 739 is the perfect addition to the fleet, much better than the 738 would have been. Its not a 'waste' or anything of the sort.